1. Field of the Invention
The present invention relates to a knock control apparatus that controls a knock occurring in an internal combustion engine.
2. Background Art
There has been an apparatus that detects a knock phenomenon occurring in an internal combustion engine by a vibration sensor (hereinafter, referred to as the knock sensor) directly installed to the block of the internal combustion engine. When a knock occurs while the internal combustion engine is running, vibrations in a specific frequency band corresponding to a bore diameter of the internal combustion engine and a vibrational mode of a knock are known to occur. The apparatus therefore detects a knock by measuring vibration intensity at this specific frequency.
There is also a knock control apparatus that suppresses a knock by correcting a spark timing to be on the retard side when a knock is detected and minimizes a torque reduction by returning a spark timing to be back on the advance side when no knock is detected. As a characteristic of the internal combustion engine, it is known that an output torque of the internal combustion engine is enhanced whereas a knock occurs readily when a spark timing is advanced and conversely an output torque of the internal combustion engine is reduced whereas a knock hardly occurs when a spark timing is retarded. This knock control apparatus therefore controls the internal combustion engine to run at a knock limit spark timing at which a torque is generated best while suppressing the occurrence of a knock by correcting a spark timing to be on the retard side when a knock is detected and by returning a spark timing to be back on the advance side when no knock is detected. It should be noted, however, that there is a case where no knock occurs when a spark timing is advanced to the extent at which a torque reaches the maximum while the internal combustion engine is run by a low load. The knock control described above is not necessary in such a running region.
In the knock control apparatus for internal combustion engine as described above, typically, there are cases where a threshold to determine the occurrence of a knock is set using a gain and an offset that are preliminarily matched to an average value of a knock signal calculated by filtering the knock signal and where the threshold is set using an average value and a standard deviation of a knock signal calculated by the filtering. However, when the running condition of the internal combustion engine changes, so do the average value and the standard deviation of the knock signal. This poses problems, such as a large number of man-hours for matching processing are involved to match the gain or the like in response to such a change and a knock is detected erroneously or left undetected because the threshold is not set appropriately.
These problems will now be described more in detail using the drawing. FIG. 7 is a view showing images used to describe a knock signal distribution in different running conditions of the internal combustion engine and knock detection by an apparatus in the related art. Herein, (1) shows a state where a knock signal V changes in response to changes of the running conditions A through C, (2) shows a distribution profile of the knock signal V in the running conditions A through C, and (3) shows a behavior of a knock determination threshold TH in a knock control apparatus in the related art in a case where the running condition changes from A to B to C.
As are shown in (1) and (2) in FIG. 7, when the running condition changes from A→B→C, an average value μ of the knock signal V changes from μA→μB→μC and a standard deviation σ changes from σA→σB→σC. In other words, as the running condition changes from A→B→C, the distribution profile D of the knock signal V changes from DA (μA, σA)→DB (μBσB)→DC (μC, σC).
A setting method of the knock determination threshold TH in the knock control apparatus in the related art will now be described. Initially, a knock signal V[n] is subjected to weighed-averaging in accordance with an equation below by interruption processing for every ignition to calculate an average value μ[n] of the knock signal V[n]:μ[n]=Kμ×V[n−1]+(1−Kμ)×V[n]where V is a knock signal, Kμ is a filter coefficient, and n is the number of interruption processing times (positive integer).
Subsequently, a variance σ[n]2 of the knock signal V[n] is calculated by weighed-averaging in accordance with an equation below using the calculated average value μ[n] of the knock signal V[n] and the knock signal V[n]:σ[n]2=Kσ2×σ[n−1]2+(1−Kσ2)×(V−μ)[n]2 where Kσ2 is a filter coefficient for variance calculation.
As is shown in an equation below, a standard deviation σ[n] of the knock signal V is calculated by calculating the square root of the calculated variance σ[n]2 of the knock signal V[n]:σ[n]=(σ[n]2)1/2.
The knock determination threshold TH is calculated in accordance with an equation below using the calculated average value μ[n] and standard deviation σ[n] of the knock signal V [n]:TH[n]=μ[n]+KTH×σ[n]where KTH is a coefficient for threshold calculation.
The filter coefficient Kμ and the filter coefficient Kσ2 for variance calculation used in the equations above are set in such a manner that in a case where the running condition of the internal combustion engine changes, the filter coefficients follow quickly in a transition period of such a change and slowly during a knock determination.
In the case of the knock control apparatus in the related art, as is shown in (3) in FIG. 7, when the running condition has changed from A to C, it is necessary to closely match the respective filter coefficients to the average value μ and the standard deviation σ that have changed to let a behavior of the knock determination threshold TH follow the changes of the average value μ and the standard deviation σ of the knock signal V. Further, as has been described, because the filter coefficients are set so that the respective filters follow slowly during a knock determination, in a case where the occurrence of a knock is determined erroneously as the knock signal V exceeds the knock determination threshold TH, there is a problem that a wrong knock determination is continued.
To eliminate such an inconvenience, there has been proposed a control apparatus for internal combustion engine configured to suppress changes of an average value and a standard deviation of a knock signal in response to a change of the running condition of the internal combustion engine by normalization (standardization, dimensionless transformation) of the knock signal (see, for example, Patent Documents 1 and 2).
The apparatuses in the related art described in Patent Documents 1 and 2 are configured to suppress changes of an average value and a standard deviation of a knock signal caused by the running condition of the internal combustion engine by normalizing the knock signal according to a typical standardization procedure expressed by an equation below using the average value and the standard deviation of the knock signal:Z=(V−μ)/σwhere Z is a post-normalization knock signal.
Patent Document 1: JP-A-2005-299580
Patent Document 2: JP-A-2005-307753
According to the apparatuses in the related art described in Patent Documents 1 and 2, a knock signal is normalized (standardized) to an average value [μ=0] and a standard deviation [σ=1] independently of the running condition of the internal combustion engine. However, because a change rate of the standard deviation σ is large at the occurrence of a knock, a vibrational component due to the occurrence of a knock is normalized, too. Consequently, a signal-to-noise ratio is lowered, which poses a problem that knock control performance becomes poor. Also, because it is necessary for normalization to calculate the standard deviation σ, calculations of a square and a square root are necessary to calculate a standard deviation σ of a knock signal separately from the knock determination threshold. This poses a problem that not only a processing load on an arithmetic device is increased, but also man-hours for matching processing necessary for calculation are increased. Further, as is shown in (3) in FIG. 7, there is a delay in calculation of an average value μ of the knock signal V when the running condition of the internal combustion engine changes. Hence, there is a problem that while the running condition of the internal combustion engine is changing, the standard deviation σ calculated from the knock signal V and an average value μ thereof becomes less accurate (it is calculated a little too large).